Printed wiring boards (also known as printed circuit boards or PWB's) are generally laminated materials comprised of two or more plates or foils of copper, which are separated from each other by a layer of nonconducting material. Although copper is generally used as the electroplating metal in printed wiring boards, those skilled in the art will recognize that other metals such as nickel, gold, palladium, silver and the like can also be electroplated. The nonconducting layer or layers are preferably an organic material such as an epoxy resin impregnated with glass fibers, but may also comprise thermosetting resins, thermoplastic resins, and mixtures thereof, with or without reinforcing materials such as fiberglass and fillers.
In many printed wiring board designs, the electrical pathway or pattern requires a connection between the separated copper plates at certain points in the pattern. This is usually accomplished by drilling holes at the desired locations through the laminate of copper plates and the nonconducting layer and then connecting the separate metal plates. Subsequently, the through hole walls of the printed wiring board are prepared for electroplating. These plated through hole walls are necessary to achieve connections between two metal circuit patterns on each side of a printed wiring board, or in addition to this, between the inner layer circuit patterns of a multilayer board.
One advantageous way of preparing the through hole walls for electroplating utilizes a liquid carbon dispersion. The steps of this process are discussed briefly below.
First, surfaces of through holes are made relatively smooth for plating. Then, the printed wiring board is preferably subjected to a precleaning process in order to place the printed wiring board in condition for receiving a liquid carbon black dispersion. After the application of the cleaner, the PWB is rinsed in water to remove excess cleaner from the board and then contacted with a conditioner solution. The conditioner solution is used to ensure that substantially all of the hole wall glass/epoxy surfaces are properly prepared to accept a continuous layer of the subsequent carbon black particles. See for example U.S. Pat. No. 4,634,691, to Lindsey, the subject matter of which is herein incorporate by reference in its entirety, which describes a suitable conditioner solution.
The liquid carbon black dispersion is next applied to or contacted with the conditioned PWB. This dispersion contains three critical ingredients, namely, carbon black, one or more surfactants capable of dispersing the carbon black and a liquid dispersing medium such as water.
The carbon black-covered board is then subjected to a step where substantially all (i.e., more than about 95% by weight) of the water in the applied dispersion is removed and a dried deposit containing carbon black is left in the holes and on other exposed surfaces of the nonconducting layer. To insure complete coverage of the hole walls, the procedure of immersing the board in the liquid carbon black dispersion and then drying may be repeated.
The carbon black covered board may optionally next be subjected to an additional graphite treatment yielding the deposition of a graphite layer on top of the carbon layer. Preferably, the carbon black-coated PWB board is first contacted with a conditioner solution, which is used to promote subsequent adsorption of the dispersed graphite particles on the carbon black layer. After the application of this optional conditioner solution, the PWB is subsequently rinsed with water to remove excess conditioner from the board.
The board may optionally next be contacted with the liquid graphite dispersion or suspension. This graphite dispersion contains three critical ingredients; namely, graphite, one or more surfactants capable of dispersing the graphite, and a liquid dispersing medium such as water. The board is then subjected to a step where substantially all (i.e., more than about 95% by weight) of the water in the applied dispersion is removed and a dried graphite deposit is left in the holes over the carbon black deposit and on other exposed surfaces of the nonconducting layer. To ensure sufficient coverage of the hole walls, the procedure of immersing the board in the liquid graphite dispersion and then drying may be repeated.
The steps of this process are described in more detail, for example, in U.S. Pat. No. 4,619,741, the subject matter of which is herein incorporated by reference in its entirety. Various modifications and refinements to this process are set forth in U.S. Pat. Nos. 4,622,107, 4,622,108, 4,631,117, 4,684,560, 4,718,993, 4,724,005, 4,874,477, 4,897,164, 4,964,959, 4,994,153, 5,015,339, 5,106,537, 5,110,355, 5,139,642, and 5,143,592, the subject matter of each of which is herein incorporated by reference in its entirety.
The carbon black and graphite dispersions on the PWB not only coat the drilled hole surfaces, which is desirable, but also entirely coat the copper plate or foil surfaces, which is undesirable. Thus, prior to many subsequent operations, all carbon black and graphite must be removed from the copper plate and/or foil surfaces.
The removal of the carbon black and/or graphite, specifically from the copper surfaces including, especially, the rims of the drilled holes while leaving the coating intact on the glass fibers and epoxy surface of the hole walls, may preferably be achieved by the employment of a mechanically scrubbing operation or a microetch or both. The microetch is preferred because of ease of use.
After the microetch step and a subsequent water rinse, the PWB may either proceed to the photoimaging process and later be electroplated or be directly panel electroplated. The PWB may be further cleaned with a citric acid or benzotriazole anti-tarnish solution or another acid cleaner solution or both after the above microetch step. The thus treated printed wiring board is then ready for electroplating operation which includes immersing the PWB in a suitable electroplating bath for applying a copper coating on the hole walls of the nonconducting layer.
Microetch solutions used to remove excess graphite and/or carbon black may be based on hydrogen peroxide or sodium persulfate as the oxidizing agent. For example, a sodium persulfate-based product may be combined with sufficient sulfuric acid to make a microetch bath containing 100–300 grams of sodium persulfate per liter of deionized water and about 1 to 10% by weight sulfuric acid.
In the case of a multilayer type board, this microetching step is especially preferred. Since, after the drying step, not only will the outer copper plate or foil be coated with carbon black but also the copper inner plates or foils exposed within the holes. Thus, the microetch procedure performs two very desirable tasks at once:                A. It removes substantially all excess carbon black and/or graphite material adhering to the outer copper plates or foils and the exposed surfaces of copper inner plates or foils in a multilayer PWB; and        B. It chemically cleans and microetches slightly the outer copper surfaces, thereby making them good bases for either dry film application or the electrolytic deposition of copper when followed by mechanically scrubbing the PWB.        
The mechanism by which this microetch works is by not attacking the carbon black material or the graphite material deposited on the copper foil directly, but rather to attack exclusively the first few atomic layers of copper directly below which provides the adhesion for the coating. Hence, the fully coated board is immersed in the microetch solution to “flake” off the carbon black and/or the graphite from the copper surfaces in the form of micro-flakelets. These micro-flakelets are removed from the microetch bath either by filtration through a pump or via a weir type filter arrangement commonly used in the PWB industry. The liquid carbon black dispersion, the liquid graphite dispersion, the microetch treatment, and the intermittent water rinses are preferably carried out by immersing the PWB in baths constructed of polypropylene or polyvinyl chloride (PVC) and kept agitated by a recirculation pump or pumped in air or by a conveyorized flood or spray machine.
These microetches have been widely used to clean carbon off copper surfaces after the application of the carbon and graphite dispersions. The etch is typically controlled at about 40 microinches, however a lower etch is desired for a variety of reasons, including the elimination of voids caused by nailheads, the reduction of negative etchback, etc.
Traditionally, the etch of the copper surface is lowered by employing one of more of the following methods: 1) less oxidant, 2) lowering the temperature of the microetching solution, and/or 3) shorter contact time. The drawback to the use of these methods is that they contribute to a less clean copper surface, thereby increasing the number of defects due to carbon residues.
Various additives for the microetching solution have been suggested to suppress the etch without hurting the cleaning capability of the microetching solution. One additive that has been used previously is citric acid. However, citric acid can leave a film on the copper surface, which can cause dry film lock (ie. inability to develop) and poor copper/copper adhesion. In addition, since citric acid is a chelator, waste treatment and disposal can be troublesome.
Thus there remains a need in the art for an additive that can suppress the etch on the surface without affecting the cleaning capability of the microetching solution.